1. Field of the Invention
The present invention relates to a speed change control apparatus for an automatic transmission.
2. Description of the Related Art
An automotive automatic transmission is generally equipped with a speed change mechanism which includes planetary gears comprised of power transmitting elements such as sun gears and planetary carriers, and which includes hydraulic friction-engaging elements such as hydraulic wet-type multiple disc clutches and hydraulic band brakes. Further, a line pressure generated by a hydraulic pump driven by a crankshaft of an internal combustion engine is used as a drive source for the hydraulic friction-engaging elements. The automatic transmission of this type controls the operating states of the friction-engaging elements by controlling the oil pressure supply to and discharge from the friction-engaging elements relating to speed change, whereby the engine torque transmission path in the planetary gear is changed to obtain a desired speed (gear ratio).
An electronic control type automatic transmission uses an electromagnetic oil pressure control valves (hereinafter called solenoid valves) to control the operating states of the hydraulic friction-engaging elements. Specifically, by controlling the on/off duty ratio of a respective solenoid valve concerned, the oil pressure supply to or discharge from an associated hydraulic friction-engaging element is controlled such that the friction-engaging element is engaged or disengaged. Further, by changing the hydraulic friction-engaging element to be engaged, that is, by engaging one of the hydraulic friction-engaging elements relating to speed change and by disengaging the other hydraulic friction-engaging element, a speed change is carried out while reducing a speed change shock upon engagement and disengagement of the friction-engaging elements.
For example, when downshifting is carried out from the fourth speed to the third speed, the friction element for establishing the fourth speed (the disengagement-side friction element) is disengaged, and the friction element for establishing the third speed (the engagement-side friction element) is engaged. By effecting the just-mentioned switching operation for the friction elements, the engine torque transmission path is changed, whereupon the downshifting operation is completed.
Generally, in the electronic control type automatic transmission, a gear position (target gear position) best suitable for the current vehicle speed and the current throttle valve opening degree is determined by referring to a shift map in which a gear position to be selected is represented as a function of the vehicle speed (the rotational speed of transfer drive gear) and the throttle valve opening degree. Based on the result of this determination, the upshift or downshift control is carried out as necessary. In either of the upshift control or the downshift control, the control is made such that the switching operation for the friction-engaging elements is completed at the moment when the rotational speed of the input shaft of the automatic transmission reaches a synchronous rotational speed for the target gear position. However, the synchronous rotational speed is higher than the rotational speed of input shaft before speed change in upshifting and lower in downshifting. For this reason, different control programs (control rules) are used for the upshift control and the downshift control.
When an accelerator pedal is depressed, the engine is in a so-called power-on state. On the other hand, when the accelerator pedal is not depressed, the engine is in a so-called power-off state. If the disengagement-side friction element is disengaged in the power-on state, the rotational speed of input shaft is increased. If the disengagement-side friction element is disengaged in the power-off state, the rotational speed of input shaft is decreased. In both of the upshift control and the downshift control, therefore, whether the engine is in the power-on state or the power-off state is determined, and a control program in accordance with the result of this determination is used. That is, different control programs are used depending on whether the engine is in the power-on state or the power-off state.
As described above, the speed change control is carried out by using four kinds of control programs: programs for upshifting in the power-on state, for downshifting in the power-on state, for upshifting in the power-off state, and for downshifting in the power-off state.
More specifically, in the power-on downshift control, the disengagement-side friction element is disengaged to increase the rotational speed of input shaft up to the synchronous rotational speed. In the power-on upshift control, the disengagement-side friction element is disengaged and the engagement-side friction element is gradually engaged, to thereby decrease the rotational speed of input shaft down to the synchronous rotational speed. In the power-off upshift control, the disengagement-side friction element is disengaged to decrease the rotational speed of input shaft down to the synchronous rotational speed. Further, in the power-off downshift control, the disengagement friction element is disengaged and the engagement-side friction element is gradually engaged to increase the rotational speed of input shaft up to the synchronous rotational speed. In any of the shift controls, when the rotational speed of input shaft coincides with the synchronous rotational speed, the engagement-side friction element is engaged completely.
A determination as to whether the engine is in the power-on state or the power-off state is made on the basis of the engine load information such as the intake air amount per one intake stroke (intake air amount information) A/N and the throttle opening degree .theta..sub.TH. If a load information sensor is faulty or a communication line for transmitting the load information is broken, therefore, the power-on/off determination based on the load information cannot be made correctly. This causes a trouble such as undesired rise in the engine rotation during the speed change or inability in speed change.
For example, in upshifting, if it is judged that the engine is in the power-off state despite the fact that the engine is actually in the power-on state, the disengagement-side friction element is disengaged before the engagement-side friction element begins to be engaged, so that the speed change mechanism of automatic transmission turns neutral temporarily. As a result, an undesired rise in the engine rotation (a sudden increase in input shaft rotational speed) occurs, so that a great shock is caused when the engagement-side friction element is engaged, resulting in remarkably deteriorated speed change feeling. In downshifting, if it is judged that the engine is in the power-on state despite the fact that the engine is actually in the power-off state, the disengagement-side friction element is disengaged, so that the rotational speed of input shaft decreases gradually. Moreover, in the power-on downshift control, the engagement-side friction element is engaged after the rotational speed of input shaft has reached the synchronous rotational speed of the target gear position. Therefore, the rotational speed of input shaft cannot reach the synchronous rotational speed, so that the engagement of the engagement-side friction element and in turn the downshifting operation is not completed.